Ronaldo is no stranger to controversy over his theatrical tumbles but the Portugal star appeared to be harshly treated by Foy at Wembley.
Television replays showed Tottenham defender Ledley King clearly made contact with Ronaldo before the United player fell to the turf in the penalty area.
Foy was quick to flash a yellow card at Ronaldo after the incident, much the bemusement of the World and European Footballer of the Year.
"If you see the replay you will see that their player kicked me in the foot," Ronaldo said.
"The referee did not make the right decision then and he didn't give me a lot, all match, I don't know why. But in every game, the referee's decision has to be respected." At least Ronaldo could console himself with United's penalty shoot-out victory.
Ronaldo missed a penalty when United beat Chelsea in a shoot-out to win last season's Champions League, but he kept his nerve this time to stroke his spot-kick past Heurelho Gomes.
He admitted he changed the side he aimed for because he thought Gomes may have seen the miss against Chelsea.
"It's always important to choose a side and put the ball there," he said.
"After Moscow I thought the goalkeeper would think I put it in the same corner but I changed."
United's Wembley win kept alive their remarkable bid to win five trophies in one season.
They already have the Club World Cup and League Cup and sit seven points clear at the top of the Premier League ahead of a trip to Newcastle on Wednesday.
United are also in contention for the FA Cup - they face Fulham in the quarter-finals on Saturday - and host Inter Milan in the second leg of their Champions League last 16 clash next week.
Ronaldo believes there is a chance United could complete a clean sweep if they remain humble.
"It is quite hard," he said. "We have to think game to game. We won this cup now, we are top of the league and still in the Champions League and FA Cup.
"With United there is always an opportunity to win everything but we need to keep our feet on the ground."